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ZOOLOGICAL SERIES 

OF 

FIELD MUSEUM OF NATURAL HISTORY 

Volume XX CHICAGO, DECEMBER 28, 1936 No. 22 

COURTSHIP AND MATING BEHAVIOR 

IN SNAKES THE LIBRARY OF THE 

BY D. DWIGHT DAVIS JAN 1 1 1937 

ASSISTANT, DIVISION OF OSTEOLOGY 

UNIVERSITY OF ILLINOIS 

Darwin, in the Descent of Man and Selection in Relation to Sex 
was the first to give point to study of the courtship and mating 
behavior of animals. Since his time much of the investigation in 
this field has been devoted to critical consideration of his evidence 
and conclusions. Recent widespread interest in manifestations of 
sex has opened up aspects of the subject that were unknown to 
Darwin. Most important among these are the genetic relations 
of sex, the role of the endocrines in the physiology of sex, as well as^f 
sexual behavior, and, of still broader interest, the fundamental 
relation between sexual behavior and the general social behavior 
of animals. 

Snakes as a group were mentioned briefly in the Descent of Man, 
but two subsequent extensive reviewers (Cunningham, 1900; Pycraft, 
1913) ignored them completely. The absence of striking secondary 
sexual characters and the extraordinary dearth of published observa- 
tions on sexual behavior in snakes are probably largely responsible 
for the general lack of interest in this subject. The recent appearance 
of an excellent and comprehensive account of the mating behavior 
of their nearest living relatives, the lizards (Noble and Bradley, 
1933), makes the meagerness of our knowledge with respect to the 
snakes particularly apparent. 

The most satisfactory approach to a study of the courtship 
behavior of animals is direct observation, preferably of undisturbed 
individuals in the field. Serious misinterpretations have not infre- 
quently arisen from observation or experimentation on captive 
material, where unnatural associations almost inevitably alter 
behavior. Unfortunately one student, no matter how much time 
he may spend in the field, can not hope to witness mating behavior 
under natural conditions in more than an extremely small number 

No. 376 257 



258 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

of animals. This results in the necessity of trying to collate the 
scattered, fragmentary observations of many individuals, with the 
attendant possibility of error. 

In attempting to work up an unusual series of observations on 
the mating of Thamnophis radix made by Mr. A. C. Weed, of the 
Field Museum staff, it became increasingly apparent that the sexual 
behavior of snakes merits more attention than it has received. 
Existing knowledge is not only comparatively limited, but it is also 
widely scattered through literature, largely in the form of brief notes. 
Many important observations are buried in the midst of unrelated 
material, where their presence would be unsuspected. No one has 
tried to correlate his work with that of others, and consequently 
few have realized what constitute the significant features of court- 
ship in this group. In order to stimulate and direct further work 
in this subject, the following summary of the literature has been 
drawn up. This is followed by a brief consideration of sexual dimor- 
phism in snakes, with particular reference to secondary sexual 
characters. I have tried also to draw a few tentative conclusions 
as to the broader implications of mating behavior in snakes. These 
cannot be regarded as final, since they rest on data that are inade- 
quate for indicating more than probabilities. 

Many individuals have assisted in the preparation of this paper. 
I am particularly grateful to Mr. A. C. Weed for making careful 
observations and extensive notes on Thamnophis radix over a period 
of several months when circumstances prevented me from witnessing 
its activities myself. Mr. R. M. Perkins, of the Zoological Society 
of St. Louis, has supplied data from personal observations without 
which the summary would have been much less complete. My 
colleague and mentor, Mr. Karl P. Schmidt, has continued to give 
unselfishly of his time and thought and I am under great obligation 
to him. Mr. John J. Janecek devoted painstaking care to the 
preparation of the illustrations. 

SUMMARY OF LITERATURE 

Naturally a major part of published observations deal with the 
most common members of the cosmopolitan family Colubridae. 
Indeed, this is true to an unfortunate degree, and disappointing 
gaps are left at important points, notably among the Elapidae, 
Viperidae, and Crotalidae. Since these include the most highly 
specialized of all snakes, they are important in the present connection. 
Through the generosity of various individuals, particularly Mr. R. M. 



5^)0.5 



1936 MATING BEHAVIOR IN SNAKES DAVIS 259 

Perkins, it has been possible to fill some of these gaps by drawing on 
unpublished notes and observations. The information supplied has 
been included in the summary given below, and has helped to make 
possible a reasonably well-rounded picture. 

Some of the observations cited are of doubtful importance, but 
have been included both for the sake of completeness and because 
the present state of knowledge does not permit intelligent elimination 
of extraneous material. 

Boidae. The members of this and the succeeding family differ 
from the more familiar snakes in possessing remnants of the posterior 
limbs. In the males these structures appear externally in the form 
of stout spurs or claws. They are present in both sexes, but are 
extremely small in females. The extraordinary part they play in 
the courtship of Constrictor constrictor is vividly described by Mole 
and Urich (1894) as follows: 

"These claws, however, are capable of being slightly protruded 
and are endowed with considerable mobility. When about to couple, 
the male extends these hooks at right angles to the body and vibrates 
them in an extremely rapid manner, scratching, as he does so, the 
back and sides of his companion. The claws scratching the scales 
of his mate make a noise which can be distinctly heard two yards 

r off. This habit has also been observed in Epicrates cenchris." 

At a later date Mole (1924) observed a similar use of the spurs 

S in the anaconda, Eunectes murinus. He states that the male "throws 
c a coil or two around the female, which is usually the larger of the 

^pair, and his claws are moved quickly and scratch the scaly sides of his 

--mate, inducing her to crawl forward slowly until union is established. 

^ One pair was thus engaged from December 24 until January 13." 

Pythonidae. Mr. Perkins has observed the use of the spurs 
in the blood python, Python curtus. The male lies close beside the 
female, with the anterior part of his body frequently lying over her. 
He then extends his spurs at right angles to his body and moves 
them slowly and rhythmically from side to side, stroking the body 
of the female with them directly over her cloaca. This behavior, 
which has been observed to continue uninterruptedly for more than 
two hours, does not produce a scratching sound, as described by 
Mole and Urich for Constrictor. Under the stimulation of this 
"tickling" the female gradually twists her cloacal region laterally 
and the male works his tail under hers. When her cloaca is nearly 
in a vertical position the female suddenly gapes it, the male inserts a 
hemipenis, and copulation takes place. 



260 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

Colubridae. The recent observations of Stemmler-Morath 
make it obvious that at least two widely different types of courtship 
behavior are found among the members of this family. Further 
work will unquestionably reveal additional behavior patterns in 
this enormous assemblage of forms. Matings have been observed 
in a number of colubrid genera, but unfortunately the meager notes 
that have been published on most of them reveal nothing that bears 
on the problem under consideration here. 

What may be designated the Natrix type of mating behavior 
has been more frequently and more completely described than any 
other. Indeed, until recently it was the only type of snake-mating 
of which there was any but the most fragmentary knowledge, and 
it has frequently been assumed to characterize snakes in general. 
Many of the important features of this type of mating were accurately 
described more than fifty years ago by Franke (1881), who apparently 
was a careful observer. His account of the mating of Coronella 
austriaca is as follows (translation) : 

"Pairing begins about the middle of April. For several years I 
kept from forty to fifty individuals in the terrarium; the males at 
pairing time exhibited much jealousy and not infrequently engaged 
in fighting, in which they bit each other. The not always willing 
female also protected herself from unwelcome attentions by biting. 
The act of copulation is like that of the common [European] water 
snake. The male crawls slowly onto the back of the female, in 
which it takes exactly the same curves in which the female happens 
to lie, and also adapts itself to any changes of position which the 
female may make. Only the anal region of the male hangs down 
toward that of the female, who recognizes the intention of the male 
by a lateral turning of the anus. During these preparations, but 
still more so during the two to three hour duration of the act itself, 
both individuals exhibit uninterrupted contractions of irregular 
strength attended by motions of the tails, which are not infrequently 
mutually intertwined. The female, in the lower position, frequently 
changes her position during this time, and may crawl some distance 
away, but not without the male, who immediately imitates every 
motion of the female. I have never observed interference on the 
part of other companions, which, in fact, crawl over or beneath the 
pair without taking any notice of them. I must emphasize that I 
have never observed a 'close intertwining' that has been described 
by other authors either in the present species or in the common 
water snake, although I have witnessed copulation more than a 



1936 MATING BEHAVIOR IN SNAKES DAVIS 261 

hundred times. Nor is a 'suitable soft substratum' sought out, 
for rough, uneven places, heaps of stones, etc., are preferred. Copu- 
lation takes place only on warm days, and most frequently during 
the mid-day sun of April or May." 

Additional features of the mating process have been noted by 
others. Truitt (1927) records an instance of a female Thamnophis 
sirtalis being followed by two males. When imprisoned in a wire 
enclosure and transported some distance, the female attracted three 
snakes of the same species to her. One of these was captured and 
found to be a male. Truitt suggests that the females, of certain 
species at least, probably attract mates by means of odors. Stemm- 
ler-Morath, on the basis of observations made in the large open- 
air terrarium at the Basel Zoological Garden, reports that he has 
observed competition among males for. the possession of a female 
only in the species of the genus Natrix among European colubrids. 
He states (translation) : 

"Only in the species of the genus Natrix does there appear to 
be in nature a kind of competition for the female. As many as 
twenty males besiege a female lying under them for hours, all lying 
in the same direction and constantly attempting to gain the back 
of the female, which is as regularly prevented by a competitor. But 
this takes place quietly, without one biting the other. As soon as 
one of the males has attained his goal, he is pulled out of the wriggling 
mass, tail foremost, by the female. The competitors who have been 
left behind, as soon as they notice the disappearance of the female, 
dash around excitedly and attempt copulation with any snake 
encountered, whether it be of the same sex or of a different species. 
The escaped pair meanwhile lies somewhere in a quiet place, and 
only the frequent convulsive movement of the tail of the male shows 
that they are alive." 

Mr. Perkins has noted a similar behavior in captive Natrix and 
Thamnophis. When a female is placed in a cage containing a number 
of males, the males are at once thrown into a state of frenzied excite- 
ment. They crawl excitedly over and among one another, and 
attempt to mate with any individual encountered, regardless of sex. 
There is no evidence of courtship in such cases. 

This striking behavior probably does not normally take place 
in nature, but is to be regarded as a result of confining a number 
of individuals in a small space under artificial conditions. Thus, 
Perry (1920) described a suggestive incident in Natrix sipedon. 
A copulating pair was encountered on a bush overhanging a stream. 



262 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

Below this pair was a "confused mass" of "at least six" other indi- 
viduals. "Presently the convulsions of the pair on the outer end 
of the bush ceased. The smaller snake slowly disengaged himself 
and joined the bunch below. His place was taken quickly by another 
and the convulsions began again. One after another, the smaller 
males paid their respects to the largest one, which I believe was a 
female." Nevertheless, as pointed out below, the probable relation 
between this aggregation reaction and the formation of snake "balls" 
cannot be overlooked. 

Dymond and Fry (1932) observed two mated pairs of the green 
snake, Liopeltis vernalis. One pair was found in shrubbery, about 
a foot above the ground. They noted that in neither pair was the 
male in contact with the female except in the cloacal region. 

Blanchard (1931) has briefly described the Natrix type of mating, 
drawing attention to two highly important features previously over- 
looked: "Before the act of copulation the male snake slides forward 
along the upper surface of the female, his chin pressed close to her 
back and his body lying along hers in sinuous curves. He then sets 
his body into rhythmic undulations. The chin tubercles are thus 
particularly well placed to serve as rubbing organs during the 
maneuver." In addition to these comments on the chin tubercles, 
he also draws attention to the fact that the supracloacal keels or 
tubercles characteristic of the males of many snakes are brought 
into contact with the body of the female during copulation. 

The behavior of other colubrids differs strikingly from that of 
Natrix and may be called the Coluber type of behavior. Stemmler- 
Morath (op.cit.) has observed a complex courtship "dance" in Coluber 
gemonensis, Elaphe longissima, and the species of Malpolon among 
European snakes. The behavior is stated to be essentially the same 
for all these species, and is described as follows for the European 
whip-snake, Coluber gemonensis (translation) : 

"A pair of splendid aesculapian snakes crawl quietly one behind 
the other. The rear animal, obviously the male, tongues the body 
of the crawling female almost continually. She gradually begins to 
move more rapidly, but the more rapidly the female glides along, 
the more actively the male follows. Finally the two long bodies of 
both snakes dash along one behind the other, almost completely 
extended. There is no longer any of the close application of the 
body to the terrain in crawling which we are accustomed to see in 
these snakes. The two animals hasten with uncanny speed through 
the area, over rocks and ponds, often up into the bushes and down 



1936 



MATING BEHAVIOR IN SNAKES DAVIS 



263 



again over the outermost twigs. This chase lasts until the male 
succeeds in reaching the side of the female. He at once throws his 
neck around the body of the female with lightning speed and draws up 
his body, coiling around her in several turns while crawling forward 
on the body of the female. The entwined pair continues to move 
about the area, and for a quarter of an hour or more they crawl 
through each other's coils uninterruptedly, so that first one and 






FIG. 28. Courtship behavior in various colubrid snakes, a, Mating dance of Coluber gemonensis 
(after Stemmler-Morath). b, Mating pair of Indian ratsnakes (Ptyas mueosus) (after Prater), e, 
Mating pair of Coluber v. carbonarius showing the neck grip characteristic of certain colubrida (after 
Schreiber). 

then the other is above. The heads are now carried always at the 
same height. Suddenly one of the animals raises its front end until 
the anterior third of the body, freed from its partner, stands almost 
vertically. The other animal does likewise, and now the heads are 
placed quite near each other and obliquely depressed forward. The 
upraised anterior parts of the body, well separated from each other, 
with only the posterior two-thirds of the bodies twined together 
like wire, resemble a lyre (fig. 28, a). But not only do they exhibit this 
handsome shape when at rest; they often dash through the terrarium 



264 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

in this attitude. In spite of the fact that they continually change 
their relative positions by their constant crawling around each 
other, they carefully preserve the character of the figure. Although 
these dances often last more than an hour, I was never successful in 
photographing it. The final union then usually takes place under a 
bush, a rock or a tree, so that one rarely is able to see it. The male 
always bites fast on the neck of the female." 

Schreiber (1912) likewise states that the male often grasps the 
neck of the female in his mouth. He also describes a unique court- 
ship behavior in Coluber v. carbonarius. He states (translation) that 
"they had mutually taken hold of each other's necks with their 
mouths, while their tails were twisted together in a spiral." When 
picked up by their tails, "their union persisted and they only let 
go from time to time with their heads, only to attempt to seize each 
other immediately, making twitching and pendulum-like back and 
forth motions with the neck." 

On the other hand, Stemmler-Morath claims that this behavior 
does not take place in Elaphe q. quatuorlineatus. In this species the 
male merely crawls quietly after the female for some time until 
copulation takes place. The male, however, often seizes the neck 
of the female in his jaws for a short time. 

The mating behavior of the Indian ratsnake, Ptyas mucosus, as 
described in a number of notes by various observers, apparently is 
similar to that described by Stemmler-Morath for the European 
colubrids. Wall (1921) quotes a letter from a Major Frere. 'Their 
bodies," he says, "were twined together and writhing, except for the 
anterior quarter. Their heads were raised from the ground for one 
foot to eighteen inches and appeared to be sparring at each other like 
two young cockerels. Their movements were active and vigorous." 
Millett (1909) described the same procedure as follows: "The smaller 
snake was slowly working his coils throughout up and down the 
other, and both snakes were keeping their heads and free part of 
their length erect and alternately twining their free part round each 
other and approaching each other's heads in a 'billing and cooing' 
manner. At the same time they were travelling very slowly in a 
forward direction." Beadon (1910) speaks of coming upon mating 
ratsnakes "with heads and free parts of the body [i.e. those not 
mutually intertwined] slightly off the ground, facing each other." 
A striking photograph of a pair of these snakes in copulation, unfortu- 
nately without comment in the text, accompanies a paper by Prater 
(1933) . This illustration, from which figure 28, b has been drawn, shows 



1936 MATING BEHAVIOR IN SNAKES DAVIS 265 

both individuals fully extended, their bodies twisted spirally around 
each other from a point a short distance behind the head almost to 
the tip of the tail. It is impossible to determine from the photograph 
whether or not the male has taken the female in his jaws. 

This behavior has not been reported in any North American 
snake, although it may be looked for in Coluber and possibly in some 
species of Elaphe. Meade (1932) in describing mating in a captive 
pair of Lampropeltis getulus holbrooki notes that the male "seized the 
body of the female in his jaws" and that "from time to time during 
the preliminaries of the mating, which continued for over half an 
hour, he would release his hold momentarily, grasping the body at a 
new place after a few seconds." 

Mr. A. S. Windsor, of the General Biological Supply House in 
Chicago, recently described to me a curious series of observations on 
the fox snake, Elaphe vulpina. Two individuals, from two to three 
feet in length, were encountered in June, 1936, during field work in a 
forest preserve area southwest of Chicago. When first found, one 
snake apparently had swallowed some four inches of the anterior 
end of the second, as evidenced by the bulge produced in the neck of 
the one by the head of the other. The bodies of both were undergoing 
continuous rhythmic and practically synchronous undulations. The 
snakes were kept under observation for about twenty minutes, and 
during all this time the undulations continued uninterruptedly. 
The first snake made no effort to continue engulfing the second, nor 
did the second seem to be alarmed by its extraordinary position. 
The snakes were definitely not in copulation. 

The observer left the scene and returned about fifteen minutes 
later. At this time the first snake no longer had its partner's head in 
its mouth, but had grasped it immediately behind the head. The 
strong muscular contractions of the first individual indicated that it 
was trying to regain the head of the second. The second snake did 
not seem to resent this aggressiveness in the least. The undulations 
of the bodies of both individuals continued without interruption 
during this time. Observation was continued for about five minutes, 
when the observer again left the scene. When he returned a few 
minutes later both snakes had disappeared. 

Elapidae. Wall (op. cit.) has recorded a few fragmentary notes 
on the mating of the cobra, Naja naja. "In Mr. Hampton's vivarium 
coitus lasted intermittently for three days. He observed that the 
pair nodded their heads continually, and their bodies quivered. 
They did not take the slightest notice of anybody in front of their 



266 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

cage. They did not expand their hoods, neither did they enwrap 
themselves around one another. Each turned the vent upwards and 
sideways to effect engagement." 

Poyntz (1927) took a copulating pair of the related sea snakes 
(species not identified) from the water in the Persian Gulf, but gave 




FIG. 29. Mating dance of Vipera berus. Crotalids assume a similar position during courtship 
(from photograph by Reuss). 

no details on their behavior. It is highly interesting, however, that 
copulation takes place in the water in these extremely aquatic snakes. 
Viperidae. It is extraordinary that nothing has appeared on the 
courtship behavior of members of this family, since the genus 
Vipera is widely distributed over Europe. A photograph by T. 
Reuss (in Heilborn, 1930) shows a pair of Vipera berus reared up in 
an attitude similar to that described below for the pit vipers (fig. 29). 



1936 MATING BEHAVIOR IN SNAKES DAVIS 267 

It is probable that the mating behavior of the Viperidae is similar 
to that of the Crotalidae, at least in its essential features, and con- 
sists of a form of courtship dance, possibly associated with movements 
by the male directed toward arousing the female. Should this prove 
to be true, the courtship dance of the extremely heavy-bodied vipers, 
such as Bitis gabonica, would be a curiously grotesque affair. 

Crotalidae. Little has appeared on the mating behavior of 
the pit vipers. It is regrettable that this highly specialized group of 
snakes should have escaped observation, since they might throw 
considerable light on the phylogeny of this behavior in the Serpentes. 
Fortunately, Mr. Perkins has been able to supply some extremely 
valuable information from observations made on captive specimens 
at the St. Louis zoo. It is possible that the mating activities of both 
the vipers and pit vipers normally take place at night, which would 
explain the lack of field observations. However, Wood (1933), 
who observed a pair of rattlesnakes courting in daylight, is inclined 
to doubt this. 

Guthrie (1927) observed several incomplete matings in a captive 
pair of Sistrurus catenatus. He records that the male was seen trying 
to copulate with the female three different times during one day, 
the female seeming "entirely passive. The tail of the male was 
wrapped entirely around the other, with the vents near together but 
not in contact. The male showed a sort of spasmodic twitching, 
mainly of the posterior half but sometimes of the entire body. At 
one time his head exhibited excitement, the tongue frequently darting 
far out and the head being thrust about above the body of the 
female, who paid not the slightest attention." 

Wiley (1929) observed several matings in a captive pair of 
Crotalus atrox. She records that the male showed particular atten- 
tion to the female "by rubbing his head and the anterior part of his 
body over her. He followed her about whenever she moved around 
in the cage and caressed her all the while. The next day the same 
thing happened. It was not until a month later that the mating 
occurred." 

Mr. Perkins has repeatedly witnessed courtship in the water 
moccasin, Agkistrodon piscivorus. Under the artificial conditions 
of captivity matings take place at all seasons of the year. Individual 
pairs in captivity seem to consort together for months. Under the 
stimulus of artificial warmth and sunlight they frequently engage in 
a characteristic courtship dance, which is highly spectacular because 
of the extreme vigor with which it is carried on by these powerful, 



268 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

heavy-bodied snakes. Both individuals erect the anterior half 
of their bodies into the air. These elevated parts of the body are 
then waved and lashed about erratically. Frequently the upraised 
parts of the pair are placed together and each snake pushes and rubs 
vigorously against its mate. The force with which these pushing 
movements are carried on is occasionally evidenced when one 
individual slips past the other. The sudden release of resistance 
under these circumstances may throw the other snake several feet 
across the cage. This extraordinary behavior may continue for 
hours at a time, and is usually followed by copulation. The posterior 
ends of the bodies are not usually twined together, although this may 
occasionally take place accidentally. Other snakes in the same cage 
do not seem to be affected by the activity of the courting pair. 

Mr. Perkins has also observed the mating behavior of captive 
specimens of the Mexican west coast rattlesnake, Crotalus basiliscus. 
The courtship of this snake is essentially the same as that of the 
water moccasin, although it is neither so violent nor so prolonged. 
The male nudges the female and rubs the side of his head against 
her body during courtship, which is a feature that has not been noted 
in Agkistrodon. 

Wood (op. cit.) momentarily observed a pair of courting Crotalus 
confluentus in the field. She states that the female was "crawling 
along the ground, and the smaller male, newly moulted, weaving 
along her back." Unfortunately this courtship was neither observed 
long enough nor described clearly enough to reveal anything of the 
normal mating behavior of rattlesnakes in the wild. 

Abnormal Mating Behavior. Several brief notes on abnormal 
matings have appeared. These are instructive in the interpretation 
of certain phases of mating behavior, and may profitably be con- 
sidered. Simultaneous copulation of one female with two males has 
been observed on a few occasions, although nothing is known of the 
frequency of its occurrence in nature. Thus, Brennan (1924) 
records an instance of two male garter snakes copulating simul- 
taneously with a single large female, and Medsger (1927) observed 
a similar case involving two males of Heterodon and the badly 
mutilated carcass of a female of the same species. Truitt (op. cit.) 
has also observed a male garter snake copulating with the carcass 
of a dead female. 

Dr. R. T. Hatt has loaned me several photographs of a captive 
mating between a male Crotalus molossus and a female Crotalus atrox. 
Nothing is known of the incidence of interspecific matings in nature. 



1936 MATING BEHAVIOR IN SNAKES DAVIS 269 

Snake ''Balls." A regular and recurrent feature of herpetological 
literature is the description of tight aggregations or "balls" of snakes. 
This subject has recently been reviewed by Wood (op. cit.), who 
doubts that the phenomenon is related to mating: "It should give 
survival value for an animal possessing internal fertilization to mate 
efficiently, and not promiscuously in hampering aggregations. An 
instinct among snakes to procreate in large numbers seems improb- 
able in animals whose physiological and anatomical development 
is well above the amphibian level." If sex recognition in snakes is 
accomplished by means of an olfactory stimulus, however, there 
is no apparent reason why an unlimited number of males should not 
be attracted to the same female. According to the observations of 
Stemmler-Morath and Perkins given above, proximity of a number 
of males to a single female may result in a mass frenzy on the part 
of the males. The phenomenon of a tight aggregation of males 
around one female might well follow without involving an "instinct 
to procreate in large numbers." Obviously the snake ball question 
can be answered definitely only by careful examination and sexing 
of the individuals making up a "ball." 

SECONDARY SEXUAL CHARACTERS IN SNAKES 

Snakes exhibit little sexual dimorphism. This is perhaps in 
keeping with the singular simplicity and uniformity of their entire 
habitus, since lizards display a wide variety of sexual differences in 
both form and color. Nevertheless, since secondary sexual differences 
frequently play a highly important part in sexual behavior, those 
that are present in the Serpentes must be regarded as important. 

True secondary sexual characters do not appear until puberty 
and frequently are concerned in some way with sexual behavior. 
The term "sexual dimorphism," on the other hand, pertains to all 
differences between the sexes, and thus includes many differences 
that are not known to be functional, or that are present from birth. 
Since the nature and origin of many of these characters are obscure, 
all known conspicuous sex differences have been included in the 
following summary. 

The total lack of striking sexual dimorphism among members of 
this group has doubtless contributed to the general lack of interest 
in the topic. Taxonomists, who have aided so much with other 
groups by cataloguing and describing sexual differences, have little 
to offer with respect to the Serpentes. Nevertheless, certain of the 
secondary sexual characters that do exist are of great interest, and 



270 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

deserve much more attention than they have received. Only twice 
has anything like a broad survey of the sexual characters of this 
group been undertaken, and both instances are of recent date. 
Blanchard (op. cit.) considered a number of North American species 
from this standpoint, and Pope (1935) has reviewed them briefly in 
Chinese snakes. 

Proportions. As in most poikilothermous vertebrates, females 
tend to exceed males of the same age in size. This is by no means 
invariably true, however, and species in which males attain the 
greater size are very erratically distributed. The functional signifi- 
cance, if any, of this disparity is not at all clear, although Pope 
(op. cit.) remarks that "since it is a well-known fact that the large 
individual females usually produce more young than small ones 
of the same species, one may safely consider body length of more 
importance to females than to males." Klauber (1936) has verified 
this correlation in the prairie rattlesnake, Crotalus c. confluentus. 
He gives a coefficient of correlation of +0.7. 

Apparently males of all species tend to have longer tails than 
females, but there is considerable overlap between the sexes in this 
respect. Thus, Burt (1928) measured 85 male and 115 female 
specimens of Thamnophis sirtalis and found that 56 per cent of the 
total fell in the overlapping group, although males exceeded females 
in the modal index as well as in the greatest absolute proportionate 
tail length. It is highly probable that the longer tail in males is 
correlated with the presence of the hemipenes and their elongated 
retractor muscles. The pronounced widening of the base of the 
tail in males is well known because of its general use as a means of 
sexing individuals in systematic work. It is obviously due to the 
presence of the retracted hemipenes. 

Scutellation. Sex-correlated variations occur in the scutellation 
of most snakes. This is conspicuously true of the gastrosteges. The 
summary of seventy-five Chinese species given by Pope (op. cit.) 
shows that males of these forms almost invariably have a higher 
subcaudal count. A few erratically distributed species show no 
variation in this respect, but in no case does the female exceed the 
male. While females tend to exceed males in number of ventrals, in 
some species there is no difference in this respect, while in others the 
males even exceed the females. 

Sexual dimorphism in the number of dorsal scale rows is much 
less common. Ruthven (1908) showed that the number of scale 
rows in any region of the body is closely correlated with body girth, 



1936 MATING BEHAVIOR IN SNAKES DAVIS 271 

and consequently he believed that females would tend to have a 
higher formula than males. Apparently this condition is seldom 
realized. Ruthven himself was unable to demonstrate any signifi- 
cant sexual difference in the enormous series of garter snakes examined 
by him. Procter (1919) likewise failed to find any sharply defined 
dimorphism in three British snakes, Natrix natrix, Coronella austriaca, 
and Vipera berus. While statistical studies of large series would 
doubtless reveal that females of many species tend to exceed males 
in numbers of scale rows, instances of complete and consistent 
dimorphism in this respect are singularly rare. In fact, the only 
snake known where this is invariably exhibited is the African colu- 
brid Bothrolycus ater, where the males have 17 rows and the female 
19 on the anterior part of the body. It is significant that in this 
species, as Schmidt (1923) has remarked, "the difference between 
the sexes is unusually pronounced, the difference in size being quite 
exceptional." 

Dimorphism in the scutellation of the head appears not to have 
been demonstrated, although a statistical study of large series of 
individuals would probably reveal its presence, at least in some 
species. 

Coloration. Sexual dimorphism in color is certainly not general. 
Intensive work on large series, however, has revealed relatively 
slight differences in color or intensity of color, or even differences in 
the pattern, in a few species. Thus Pope (op. cit.) was able to dis- 
tinguish sexual differences of this character in Natrix annularis, 
Pseudoxenodon bambusicola, Psammodynastes pulverulentus, Trimere- 
surus monticola, and T. stejnegeri, among seventy-five Chinese snakes. 
Of these, only Trimeresurus stejnegeri showed any marked dimor- 
phism, the others exhibiting only increased intensity of color in one 
or the other sex. Among European snakes, Coronella austriaca and 
Vipera berus have long been known to present a more or less constant 
sex dichromatism (Franke, Schreiber, Durigen, Boulenger). Never- 
theless, a hue or pattern characteristic of one sex is decidedly the 
exception rather than the rule among snakes. Whether any of those 
that do occur make their appearance only with the attainment of 
sexual maturity, and would thus be associated directly with the 
hormones of the gonads, is unknown. 

Assumption of a nuptial coloration by the male is wholly unknown. 
In this respect snakes stand in striking contrast to lizards, among 
which nuptial livery is widely distributed. 



272 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

Chin Tubercles and Anal Keels. The sexual differences described 
above are instances of sexual dimorphism, but can hardly lay claim 
to recognition as true secondary sexual characters, since there is no 
evidence that they have any functional significance or that they are 
correlated with the onset of puberty. In some snakes, however, 
there are structures which play a definite role in courtship. In 
sexually mature males of certain species a cluster of tubercles is 
present on the symphysial region of the chin (fig. 30). The presence 
in males of ridges or knobbed keels on the dorsal scales surrounding 
the cloacal region is even more widely distributed. Blanchard (1931) 






FIG. 30. Chin region in male snakes, showing the distribution of tubercles and pustules. The 
pustules in Thamnophis are not as prominent as indicated in the figure, a, Natrix rhombifera. b, Ninia 
sebae. e, Thamnophis radix. 

has recently examined a variety of North American snakes with 
reference to these structures. He found that they are developed in 
Carphophis, Diadophis, and Natrix after sexual maturity is attained. 
An insignificant number of females also possessed anal keels, but 
whether or not their presence was correlated with abnormalities in 
the gonads was not determined. Harrison (1933) obtained similar 
results with Thamnophis. 

Blanchard found chin tubercles only in Natrix rhombifera among 
North American snakes. From an examination of nearly two 
hundred male specimens of this species, of various ages and collected 
at various seasons, he was able to determine that these structures 



1936 MATING BEHAVIOR IN SNAKES DAVIS 273 

make their appearance only with the attainment of sexual maturity, 
and that they are not subject to seasonal hypertrophy. Pope 
(op. cit.) identified them in eight species of Chinese snakes of the 
genera Natrix, Ophisthotropis, Enhydris, and Amblycephalus. They 
are well developed in the Central American snake Ninia sebae 
(fig. 30, &), and further work will doubtless reveal their presence in 
other species. These facts show that the distribution of these 
structures is erratic in the extreme. Their sporadic occurrence bears 
no relation to natural affinities. It seems likely, therefore, that they 
have evolved independently on several occasions. Observations on 
the courtship behavior of species bearing these structures are lacking, 
so that suggestions as to their functional significance must be 
tentative and inferential. 

Chin tubercles are not present in Thamnophis. Comparison of 
the region in the two sexes, however, shows that the small sensory 




FIG. 31. Modification of the anal keels in male Aspidura trachyprocta. a, Posterior end of body, 
showing distribution of hooks. 6, A single scale, enlarged, seen from the side. 

pustules characteristic of many snakes are considerably hypertro- 
phied in male garter snakes (fig. 30, c). 

This is also true of some other North American colubrids ( Natrix, 
Storeria). In others (Coluber, Elaphe, Pituophis), however, pustules 
are present but are equally developed in both sexes. There can be 
little doubt that the slightly hypertrophied sensory pustules in the 
male Thamnophis correspond to an early stage in the phylogenetic 
history of the tubercles in other snakes. 

Noble (1934a) has recently examined both the chin tubercles and 
the anal knobs histologically. He states that "each organ consists of 
a capsule of irregularly arranged tactile cells and many nerve fibers. 
The organs of the chin differ radically from those above the cloaca 
in having one or more narrow extensions piercing the overlying 
epidermis to near the horny layer." 



274 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

Spurs in Boas and Pythons. There is considerable sexual dimor- 
phism in the size of the claw or spur which is the only external 
vestige of the hind limbs in boas and pythons (Beddard, 1905). 
In the males of many species this structure is a comparatively large, 
curved hook, whereas in the female it is often reduced to a tiny 
horny projection (fig. 32). It is not known whether the hyper- 
trophy of this structure in males is correlated with the onset of 
puberty, and the material at hand is not extensive enough to de- 




l/est/<?/a/ Spur 

FIG. 32. Sexual dimorphism in t he spurs of a boa, Epierotet st riot us. a, Male. 6, Female. 

termine this by an examination of series of immature and adult 
specimens of both sexes. These claws deserve considerable atten- 
tion, since they are among the few structures in snakes which 
function actively in courtship. 

Anal Scent Glands. Both sexes of snakes are provided with a pair 
of glands buried in the musculature of the tail. These glands pour 
into the cloaca a pungent secretion, the odor and color of which 
vary considerably from species to species. This secretion has often 
been associated with the sexual behavior of snakes, but since the 
glands are present and equally functional in both sexes, as well as 
in young individuals, its sexual significance is doubtful. The expul- 



1936 MATING BEHAVIOR IN SNAKES DAVIS 275 

sion of this liquid is important in the defensive behavior of many 
species, and it is at least possible that it may function in recognition 
of species. Its true function, therefore, is obscure. 

COURTSHIP AND MATING IN THAMNOPHIS RADIX 

The plains garter snake, Thamnophis radix, is the commonest and 
most generally distributed snake in the Chicago region. It is a 
hardy species, varying its staple diet of earthworms with a wide 
assortment of foods, and thriving in a variety of habitats. It is not 
uncommon in well-populated suburban areas, where "rock gardens" 
constructed by amateur horticulturists offer it a congenial retreat. 
If unmolested, one or more individuals usually take up their abode 
in these loose piles of rock, where they seem to remain from year 
to year. 

The observations forming the basis of the following discussion were 
made by Mr. A. C. Weed, of the Field Museum staff. Mr. Weed, 
an observer of long and wide experience, made voluminous notes 
on each of the three days when mating activities were observed. 
He has generously placed these notes in my hands and they have 
been amplified by personal discussions the day following the actual 
observations. 

The observations were made in the yard of Mr. Weed's home at 
Morgan Park, Illinois. The arrangement of this area is shown in 
figure 34. The snakes had apparently been living in the rock garden 
for several years, retreating into it to hibernate in the fall and 
reappearing in the spring. They had not been seen in 1936 before 
the initial observations were made, on April 19. The individuals 
were not marked, but were readily identified on any given occasion 
through differences in size and coloration. 

Observations of April 19, 1936. The snakes concerned in the 
observations of April 19, two males and a female, were first 
seen in the rock garden shortly after 10:00 A.M. The day was bright 
and sunny, with a chilly southwest wind and an air temperature 
between 50 and 60 F. The larger of the two males, estimated to 
be about twenty-four inches in length, was brilliantly colored and 
had apparently shed recently; the colors of the smaller male, which 
was about six inches shorter, and of the female were comparatively 
dull. The female was approximately the same length as the larger 
of the two males. 

The female and the smaller male were first seen together on top 
of the rock garden. They were then very alert and cautious and 



276 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

disappeared quickly. A few minutes later they appeared together 
in a mass of dry leaves about three feet north of the rock garden, 
where they were sheltered from the wind and in the bright sun. 
They were still very alert and active, moving in and out among 
the stones. The male at this time showed no active interest in the 
female, although he remained near her. Within a few seconds the 
third snake, immediately recognizable by its brilliant colors, appeared 
for the first time and also entered the rock pile. A moment later 
the heads of all three snakes were in sight, protruding from crevices 
between the stones. 

After half a minute of inactivity the female suddenly darted 
from the stone pile, with the smaller male in close pursuit. Moving 
north about three feet, she described a circle about two feet in 
diameter, finally stopping with her body nearly straight. The 
male was two or three inches behind. After a short pause he touched 
the female with his muzzle several inches from the tip of her tail, 
apparently directly over the vent. He hesitated momentarily, 
then started to glide along her back, following each curve of her 
body closely. During this movement the male's neck was arched 
slightly, thus bringing the symphysial region of his chin directly 
in contact with her dorsal midline. No flicking of the tongue on 
the part of the male was observed, and the female remained motionless. 

When the tails of the pair were together the male stopped and 
began to bend the rear part of his body around hers. Contact was 
apparently established almost immediately. A few seconds later 
there was a gradual tensing and quivering of the entire body of the 
male. This movement brought the cloacal regions of the pair into 
very intimate contact. About five seconds later the body of the 
male suddenly relaxed slightly and became quiet. These actions, 
which were interpreted as indicating the actual orgasm, were 
repeated at intervals of less than a minute at first, with later con- 
tractions spaced somewhat farther apart. After a dozen or more 
contractions of his body, the male withdrew the hemipenis, turned 
his body, and seemed to make a new entry with the opposite hemi- 
penis. The female was quiet during the first few orgasms, although 
during the later ones her body tensed and quivered in the same way 
as that of the male. After about a dozen orgasms in the new position 
the pair separated voluntarily and the male moved away. 

During these activities the second male emerged from the rock 
pile. Moving up to the mated pair, he ran his chin along the back 
of the female exactly as described for the first male, but made no 



1936 



MATING BEHAVIOR IN SNAKES DAVIS 



277 



attempt to copulate until the original male had withdrawn and 
reinserted, when he tried half-heartedly to maneuver his cloaca 
around to that of the female. At this time he emitted a small quan- 
tity of whitish, semi-liquid material, apparently the product of the 
scent glands, although its identity was not determined. 

As soon as the first male started to move away the second renewed 
his attempt to copulate. The first returned quickly, however, and, 




FIG. 33. Copulating pair of Thamnophia radix. Note the position of the head of the male (from 
photograph by Blanchard). 

nosing his way under the other male, mated for the second time. 
Ten minutes later the three snakes were lying quietly near one 
another. At a slight disturbance the first male left. The second 
male immediately went through the courtship activities, running 
his chin along the back of the female from a point near the base of 
her tail until he had reached a position suitable for mating. Copula- 
tion took place at once, following the same pattern as that described 
for the first male. The hemipenes were employed successively as 
before. The first male returned and tried to effect an entry from 
the opposite side, but was unsuccessful. 



278 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

Five minutes later all three snakes had moved so that their 
bodies were completely hidden in the leaves and grass, leaving only 
their tails visible. The tails were side by side, and were moving, 
in unison, quite rapidly from one side to the other, as if both males 
were trying unsuccessfully to copulate at the same time. A few 
minutes later the snakes were lying near one another and, contrary 
to their behavior a short time before, were very alert. When the 
observer approached they all beat a hasty retreat and hid in the 
rock pile. 

These activities occupied approximately an hour. During the 
time of active courtship and mating, none of the snakes paid any 
attention to the observer's presence, allowing a cautious approach 
to within four or five feet without taking alarm. This behavior 
was in sharp contrast to their wariness immediately before and after 
mating activity. The dominance of the smaller of the two males 
over his larger companion of the same sex was striking and con- 
sistent. Although there was no evidence of active antagonism or 
pugnacity between them, the larger male seemed to recognize the 
dominance of his smaller rival and courted the female actively only 
when his companion was not present. 

Observations of May 10. A remarkable series of observations 
made on May 10 leaves little doubt that the sense of smell may 
play an important part in the mating of Thamnophis radix. They 
took place in the same spot as those of April 19, and under similar 
circumstances. The female was a smaller individual than the 
female of April 19. 

May 10 was cloudy, with an air temperature of 76 F. The 
pair was first seen, in copulo, at 9 :40 A. M. They were left undisturbed 
for ten minutes, during which time they remained in connection. 
Then the observer approached cautiously and the female left quickly 
when he had reached a distance of three feet. The pair apparently 
broke readily, for the male was not dragged by the female, as has 
frequently been described. When the observer approached to 
within six inches the male started to move, but stopped about the 
time his body was straightened out from the loosely coiled position 
it had occupied. He waited in this position for about twenty seconds, 
then obviously began to search for the trail of the female. He 
moved roughly in a circle, and made almost a complete circuit of 
the place where the pair had originally been before he struck her 
trail. During this circling he continually flicked the ground with 
his tongue, touching almost the whole upper surface of the forked 



1936 



MATING BEHAVIOR IN SNAKES DAVIS 



279 



part, with the tips directed backward, to the ground each time. 
When he struck the trail of the female the tongue-flicking became 
noticeably more rapid. The subsequent movements of the snakes 
are shown somewhat diagrammatically in figure 34. 

As soon as he found the trail, the male followed it very exactly 
into a clump of flowers, a distance of four feet. The snakes were 
lost to the observer for a moment, but were shortly seen to emerge 
from another clump of flowers four feet away. The female immedi- 
ately started east toward a large clump of dense underbrush where 




Elder Underbrush 

',4/ye? Bright <t Clump (mainly hollyhock) 



Wire fence 



/ 



SnV of female 

^ Srail of male 

IIIIIIIIIIIHIIII Course offuhen definitely trailing ? 

" Course of duke* probably trailing ? 

X Points uhere f struck brail of- J 
Scale: finch -3 f I. 



FIG. 34. Diagram showing movements of courting garter snakes (see text). 

she could not have been followed by the observer. She was headed 
off, and turned northwest. She then moved rapidly into another 
large clump of underbrush along the fence, into which she dis- 
appeared. A few seconds later she was seen to emerge from the 
farther side of this clump. The female was lost permanently to 
the observer at this point, but from subsequent behavior of the 
male it is apparent that she continued moving along the fence, 
probably eventually taking refuge in the rock garden. 

Meanwhile the male lay quietly in the open. A moment later 
he started to crawl toward the underbrush from which the female 
had been headed off. Circling through the vegetation several times, 



280 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

he eventually struck the trail of the female again. The male was 
then crawling at a right angle to the trail. Turning abruptly, he 
followed it directly at full gliding speed until close to the second 
clump of underbrush. Here he apparently overran the "scent" 
on a slight turn. Circling about, he picked it up again on the first 
cast, and apparently followed it straight through. 

Continuing along the fence for about ten feet, he came upon a 
large female T. radix, apparently the one of April 19. Just before 
this he must have passed unobserved very close to another snake, 
which was probably the "bright colored male" of April 19. His 
path brought him to the body of the female about at her vent. He 
stopped momentarily and seemed to press his chin down on her 
back, then passed on quickly. The observer at this time was about 
fifteen feet away, and the details of the behavior could not be seen. 
The male continued along the fence for about ten feet farther, and 
then seemed to lose the scent. He circled under the fence, and 
after casting about several times struck the trail he had just left 
and started to backtrack along it. He followed the trail until he 
again encountered the "large female." Again he stopped and 
pressed his chin along her back, moving toward her head for about 
an inch in the same way as described in the courtship of April 19. 
He did not continue courting this female, however, but left abruptly. 
Circling the rock garden several times, he finally entered it and 
disappeared at 10:10 A.M. 

The "bright male" disappeared shortly after the male under 
observation had passed, but the "large female" remained in the 
same place for an hour longer. When the observer approached, 
she appeared nervous and disappeared. 

The mating activities just described occupied approximately 
thirty minutes of elapsed time. On several occasions there was 
unmistakable evidence that the male was tracing the female wholly 
by means of some chemical sense, presumably smell. She was 
certainly invisible to him from the time the observations began. 
On three distinct occasions he was seen to search for her trail deliber- 
ately, and twice when he struck it he turned abruptly at a right 
angle to his former course. On a fourth occasion the male gave 
every evidence of picking up the trail of the female, although she 
was not seen by the observer to pass that point. Each of these 
points is marked by an x on the diagram (fig. 34). It has frequently 
been stated that the tongue in snakes serves to carry odorous par- 



1936 MATING BEHAVIOR IN SNAKES DAVIS 281 

tides to the olfactory organs, and in this connection the continuous 
flicking of the tongue by the searching male is particularly interesting. 
The male's lack of interest in the "large female" is likewise note- 
worthy, indicating either that snakes are able to distinguish and 
identify individuals of their own species, or that the female during 
courtship gives some sign of acceptance or the opposite to the court- 
ing male. 

Observations of May 24- Observations were made on this date 
which supply further data on the mating behavior of this species. 
A close watch on subsequent week-ends failed to reveal further 
mating activity, although the snakes were seen regularly. The 
1936 breeding season in Thamnophis radix therefore began about 
the middle of April and continued into the last week of May. 

May 24 was warm and mostly cloudy, with an air temperature 
of about 80 F. The area around the rock garden was examined every 
half hour or oftener during the morning, but the snakes were not 
seen until 12:00 M. Three individuals, two males and a female, 
were observed. These were apparently the same snakes as those 
seen on April 19. 

When first observed, one of the males was already in copulation 
with the female. The smaller male was trying vigorously to dis- 
possess his rival by forcing his head and body between the mated 
pair. The movements of the unmated male agitated the copulating 
pair considerably, causing the copulating male to lose contact with 
the female several times. All three snakes were much more active 
than on previous occasions, and the female in particular seemed 
much more alert. The three snakes were so closely intertwined 
at all times that it was difficult to be certain, except two or three 
times, which male was actually in copulation with the female. The 
smaller male seemed to be unsuccessful at all times, however, in 
spite of his greater activity. During the entire time of certain 
contact, the larger male kept the whole lower surface of his head 
pressed against the back of the female, with its longitudinal axis 
parallel with her vertebral column. The smaller male also assumed 
this position a few times. 

When the snakes were first seen, the smaller male was moving 
rather actively, and once was as much as three inches away from 
the other two. Later he twined in with the others and the three 
moved as a unit until they disappeared. The males seemed to pay 
no attention to the observer, but the female watched him very 
closely when he sat down within a distance of four feet. When he 



282 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

returned after an absence of several minutes the three snakes had 
moved forward leaving no more than half their bodies exposed. The 
heads could be seen occasionally on the far side of one of the stones. 
They continued to move forward slowly, and within ten minutes 
had disappeared completely. 

The behavior of these three snakes suggests a possible explanation 
of the formation of "snake balls." If two males will besiege a female 
simultaneously, what is to prevent this number from increasing, 
when additional males are present, until the writhing aggregations 
of snakes described in the literature are formed? 

SEX RECOGNITION IN SNAKES 

It has been realized only rather recently that sex recognition 
may have an important bearing on courtship behavior. In many 
animals, such as lizards and birds, striking secondary sexual charac- 
ters and complicated "courtship" displays are now known to be 
used to intimidate other individuals, regardless of sex, rather than 
to court the female. Mating is attempted with any individual that 
can be forced to submit, and sex recognition as such appears to be 
absent. In other animals, as for example frogs, when a male attempts 
to mate with another male, the second individual gives a warning 
note which serves effectively to warn the aggressive male of his 
mistake. The female fails to make this warning cry, so sex recogni- 
tion operates only between males. On the other hand, it has long 
been known that a female moth may attract hundreds of males 
by means of a chemical sense. In other words, the males recognize 
the female as distinguished from other males of the same species. 
In view of the voluminous literature on sex recognition in various 
other groups, it is strange that nothing has been written that bears 
directly on it in snakes. Experimental work aimed directly at this 
subject is much to be desired. Certain tentative conclusions may, 
however, be drawn from existing data. 

The observations recorded above (p. 278) indicate that odor is 
an important factor in enabling the male to seek out and trail the 
female. From this it seems likely that the same factor is involved 
in enabling the male to distinguish between the sexes at the time 
of mating. If this is true, it is a method of sex identification that is 
unknown elsewhere among the lower vertebrates, and is strikingly 
similar to that employed by certain moths. Noble and Clausen 
(1936) have recently corroborated these observations experimentally. 
In studying the factors responsible for the aggregation behavior 



1936 MATING BEHAVIOR IN SNAKES DAVIS 283 

of Storeria, they tested the ability of snakes to "trail" other indi- 
viduals across the bottoms of glass-sided cages. The body integu- 
ment or the cloacal region, respectively, were rubbed along marked 
trails, and the reactions of other individuals, when released in the 
cage, observed. They conclude that "it may be stated that nor- 
mally, irrespective of the breeding season, there is some odorous 
substance given off by the integument of these snakes which attracts 
other members of the same species, and that during the breeding 
season this, or more probably another substance, is given off by the 
female which in turn attracts the male and enables the male to 
distinguish sex. The nature of this substance is as yet undeter- 
mined." Curiously enough, they state that snakes not only failed 
to follow a trail scented with the cloacal gland secretion, but even 
avoided it; they say "the body integument and not the cloacal 
glands leave a trail which aids snakes in seeking and identifying 
the opposite sex" (p. 308), and "in all cases the snakes failed to 
cross a trail scented with cloacal gland secretion" (p. 309). It is 
difficult to see how a snake crawling along the ground could fail to 
bring its cloaca into contact with the substratum, and in so doing 
leave a trail of scent from that part of the body, yet we have seen 
(p. 278) that a courting male Thamnophis will follow such a trail 
assiduously. 

It is apparent, then, that snakes recognize sex directly, through 
an olfactory stimulus. In this they stand in very striking contrast 
to lizards, where sex is recognized by the trial and error method of 
attempting to mount any individual, regardless of sex, until a female 
is found with which mating can be accomplished. 

DISCUSSION 

The literature reveals a need for critical observation of the 
mating behavior of even the commonest species of snakes. Obviously 
there is not a simple, stereotyped pattern broadly characteristic 
of the entire group, as heretofore has been generally supposed. On 
the contrary, further investigation promises to reveal an extraor- 
dinarily rich variety of characteristic performances. Detailed 
knowledge is now available only for a few of the most common 
colubrids, although something is known of courtship behavior in 
nearly all the important families of snakes. Without attempting to 
theorize on the basis of a few genera which seem to supply favorable 
data, certain conclusions, some of them admittedly tentative, may 
be drawn. 



284 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

It has long been known that certain definite courtship patterns 
characterize natural groups of species in some invertebrates, in 
salamanders, and in birds. Noble and Bradley (1933) have shown 
that this is also true to a certain extent among lizards. In the 
Serpentes, data for more than one or two genera of a family are 
available only in the Colubridae. There the courtship pattern of 
Thamnophis, which consists of rubbing movements by the male 
directed toward self-stimulation, with the female remaining entirely 
passive, is wholly different from the complicated "mating dance" 
of Coluber, Elaphe, Malpolon, and Ptyas, which is apparently an 
expression of mutual excitement. Whether the simpler procedure 
that has been described for Lampropeltis and certain species of 
Elaphe, in which the male merely grasps the female in his jaws, 
represents another modification of the behavior pattern or is merely 
incompletely observed or abnormal courtship, is uncertain. It 
seems likely that further observation will reveal that identical or 
closely similar performances tend to characterize natural groups. 
It is, of course, impossible to say to what extent this may be true. 
A simple courtship behavior perhaps is associated with more "primi- 
tive" species. In this connection it is suggestive that Natrix and 
closely related genera are regarded as more primitive than Coluber 
and its close relatives. 

Data are extremely fragmentary for the non-colubrid families. 
The courtship behavior of boas and pythons is apparently unique. 
Indeed, because of the absence of well-developed external limb rudi- 
ments in other snakes, it could not be duplicated. Movements of 
the hind limbs form an essential part of the courtship pattern in 
many lizards. Although there is small probability of any correlation, 
this fact is interesting in view of the position the Boidae and Pytho- 
nidae occupy at the bottom of the evolutionary scale in snakes. 

A mating dance takes place in all poisonous snakes upon which 
observations have been made. This dance is so strikingly similar 
to the Coluber type of dance as to suggest phylogenetic continuity, 
although in some respects the Coluber type is more complex than 
that of the morphologically more specialized poisonous snakes. In 
this connection, knowledge of the mating behavior of members of 
the annectant Boigidae might prove illuminating. 

Extreme development of those secondary sexual characters not 
directly concerned with the act of copulation (horns and other 
appendages, hypertrophied or brilliantly colored parts of the body) 
is frequently associated with fighting or intimidating displays among 



1936 MATING BEHAVIOR IN SNAKES DAVIS 285 

males. Such activities are directed either toward acquisition of a 
harem or defense of territory, or they are the means of sex recog- 
nition. Harem formation is highly characteristic of certain gregari- 
ous mammals, birds, and at least one lizard (Amblyrhynchus). 
Defense of territory during the breeding season occurs frequently 
among birds, and has been noted in several lizards. Sex recognition 
through intimidating displays is likewise characteristic of certain 
members of these two groups. 

Fighting and development of weapons for conflict or intimidation 
are both conspicuously absent in snakes. Such sex-limited structures 
as are present function solely as stimulating organs. Aggregations 
of many individuals are known to take place during hibernation in 
Coluber, Thamnophis, Storeria, and Crotalus, and probably occur in 
many others, but snakes are in no sense gregarious. Mating nor- 
mally takes place at the time of emergence from hibernation before 
the population has scattered, but in spite of the resulting close 
association of many individuals fighting does not seem to occur. 
Franke alone remarks that captive breeding males of Coronella "not 
infrequently engaged in fighting, during which they bit each other." 
Farther on he remarks, however, that he has never observed other 
individuals interfering with a mated pair. Other authors have been 
at pains to emphasize the absence of fighting. 

This lack of fighting is probably correlated with the failure of 
males to defend territories or harems. The absence of armature or 
intimidating devices, in turn, is related to the failure of males to 
fight. The whole social life of snakes appears extremely simple in 
comparison with that of many other groups, yet the courtship 
behavior of these animals is curiously diversified, and in some 
respects rather complex. 

In many respects the Natrix-Thamnophis group occupies a 
unique and anomalous position. The sexual behavior of these 
snakes is curious, even degenerate, compared with that of other 
snakes. Courtship is of a type unknown elsewhere. There is no 
indication that the female participates in it, except as a purely 
passive object of the attentions of the male, nor does the male behave 
in such a way as to stimulate her. There is no evidence that indi- 
viduals recognize each other, whereas in other snakes pairs are 
known to consort together almost permanently. So-called polyandry 
has often been described in both Natrix and Thamnophis, and prob- 
ably normally takes place when more than one male is in the vicinity 
of a breeding female. Strangely enough, this aberrant type of 



286 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

mating behavior has generally been assumed to be typical of the 
snakes as a whole. 

MATING OF SNAKES AND LIZARDS COMPARED 

While the Serpentes and the Sauria form two sharply distinguished 
groups of reptiles, they are conceded to have had a common ancestry, 
and are far more closely related to one another than either is to 
any other reptilian line. It is therefore of more than ordinary inter- 
est to compare the mating behavior of members of these two groups. 

The behavior of lizards has been studied in detail by Noble 
(1934b), Noble and Bradley (1933), and Evans (1935, 1936), mostly 
through observations on captive individuals. Noble and Bradley 
concluded that active sex recognition does not exist in lizards, but 
Noble (1934b) believed that sight might be a factor in Sceloporus 
after observing it in the field. Noble and Bradley concluded also 
that such sex discrimination as does exist is based on the fight reac- 
tion displayed by the males when approached by other individuals. 
In other words, copulation is attempted with any individual that 
can be forced to submit. Evans (1935) believed, on the other hand, 
that sight is an important factor in sex recognition in Anolis, and 
that homosexual matings are probably induced by captivity. The 
direct recognition of sex, through scent, which is found in snakes, 
stands in the sharpest possible contrast to that of lizards, whether 
it is indirect or by sight in the latter. 

Noble and Bradley concluded that the courtship and mating 
of lizards is "a very stereotyped performance, which has changed 
slowly in phylogeny." The savage rape, with the female in active 
flight, which is practiced by male lizards, seems a very primitive 
type of behavior when compared with the mutual mating dance of 
many snakes, or the persistent, gentle persuasion indulged in by 
male boids and pythonids. The varied basic courtship patterns 
of snakes show that courtship in this group is by no means stereo- 
typed, but has undergone radical and significant changes in the 
history of the group. 

One of the most striking features of the behavior of lizards is the 
assuming and defending of territories by males. This practice, 
which is well known in birds, is probably closely correlated, or even 
identical, with the "despotism" or "dominance" noted by Evans 
(1935) in captive populations. In some lizards at least (Anolis, 
Sceloporus}, it is in this connection that the secondary sexual charac- 
ters of the male are brought into full play. Such sex-limited struc- 



1936 MATING BEHAVIOR IN SNAKES DAVIS 287 

tures are invariably either true weapons or highly colored parts 
which are used in bluffing displays. There is no known structure 
in lizards which has a hedonic or stimulating function. It is note- 
worthy that snakes, on the other hand, are not known to define and 
defend territories, and fighting does not seem to be a normal accompa- 
niment of their behavior. Secondary sexual characters are extremely 
limited among them. Highly ornamented parts, which are so 
common in male lizards, never occur. Both of the known secondary 
sexual characters in snakes (hypertrophied spurs in boas and pythons 
and chin tubercles in certain colubrids) function directly as stim- 
ulating organs. The same is probably true of the anal knobs or 
keels found in the males of many snakes. 

As investigation progresses, it is increasingly apparent that 
the sexual behavior of lizards is strikingly similar to that of birds. 
This can readily be accounted for on the basis of the reptilian ancestry 
of birds, if the snakes are disregarded. Whether snakes have acquired 
unique behavior patterns along with their structural specializations 
is, of course, a question which cannot be answered. The indications 
are, however, that this may be true. 

Under any circumstances, it is obvious that the mating behavior 
of snakes differs from that of lizards in every essential respect. The 
implications of this fact cannot be fully evaluated at this time, 
although they may well prove to be of great significance. 

SUMMARY AND CONCLUSIONS 

(1) Several distinct types of courtship are evident in snakes. 
In boas and pythons, the male arouses the female by scratching 

her body, above the cloaca, with his spurs. 

In the Natrix group of colubrids, the male rubs his chin along the 
back of the female, which results in self-stimulation of the male. 

In the Coluber group of colubrids, there is a mutual mating 
"dance." 

In the Crotalidae, and probably also in the Elapidae and Viper- 
idae, there is a "dance" similar to that of Coluber. 

(2) Available data indicate that given types of courtship 
behavior are common to related groups of species. 

(3) Secondary sexual characters are almost unknown among 
snakes. The chin tubercles and anal keels or knobs found in a few 
species and the hypertrophied spurs in male boas and pythons are 
the only ones known to function in courtship. 



288 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XX 

(4) Sex recognition depends on the sense of smell. The males, 
of certain species at least, may trail the female for a considerable 
distance at the time of mating. Odor apparently emanates from 
the body integument, rather than from the scent glands (Noble and 
Clausen). 

(5) Males do not fight among themselves during the breeding 
season. Correlated with this, there is no defense of territory or 
of mates. 

(6) The courtship of lizards is in many respects similar to that of 
birds. On the other hand, the mating behavior of snakes is of a 
wholly different type. It does not closely resemble that of any other 
group of vertebrates. 

(7) There is at present little evidence in favor of sexual selection 
in the mating behavior of snakes. 

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UNIVERSITY OF ILLINOIS-URBANA